The present invention relates to broad-spectrum visible light-emitting phosphors having a garnet structure activated with rare-earth metal ions. In particular, the present invention relates to a terbium aluminum oxide garnet phosphor activated with cerium that emits yellow light under blue-light excitation. The present invention also relates to white light sources using these phosphors.
A phosphor is a luminescent material that absorbs radiation energy in a portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. Phosphors of one important class are crystalline inorganic compounds of very high chemical purity and of controlled composition to which small quantities of other elements (called xe2x80x9cactivatorsxe2x80x9d) have been added to convert them into efficient fluorescent materials. With the right combination of activators and inorganic compounds, the color of the emission can be controlled. Most useful and well-known phosphors emit radiation in the visible portion of the electromagnetic spectrum in response to excitation by electromagnetic radiation outside the visible range. Well-known phosphors have been used in mercury vapor discharge lamps to convert the ultraviolet (xe2x80x9cUVxe2x80x9d) radiation emitted by the excited mercury vapor to visible light. Other phosphors are capable of emitting visible light upon being excited by electrons (used in cathode ray tubes) or x rays (for example, scintillators in x-ray detection systems).
The efficiency of a lighting device that uses a phosphor increases as the difference between the wavelength of the exciting radiation and that of the emitted radiation narrows. Therefore, in the quest for improving efficiency of white light sources, effort has been dedicated to finding a source of stimulating radiation that has wavelengths longer than that of UV radiation and phosphors that respond to those wavelengths. Recent advances in light-emitting diode (xe2x80x9cLEDxe2x80x9d) technology have brought efficient LEDs emitting in the near UV-to-blue range. The term xe2x80x9cnear UVxe2x80x9d as used herein means UV radiation having wavelengths in the range from about 315 nm to about 400 nm. These LEDs emitting radiation in the near UV-to-blue range will be hereinafter called xe2x80x9cUV/blue LEDs.xe2x80x9d As used herein, a UV/blue LED may emit radiation having wavelengths in the near UV range, in the blue light range, or in a broad range from near UV to blue. It would be an advance to the technology of lighting to provide a range of phosphors that can be stimulated by the radiation emitted from these UV/blue LEDs radiation sources to allow for a flexibility in the use of phosphors for generating various color LEDs. Such phosphors when combined with the emission from the UV/blue LEDs can provide efficient and long lasting lighting devices that consume little power.
Many near UV/blue LEDs based on combinations of nitrides of indium, aluminum, and gallium have recently appeared. For example, U.S. Pat. No. 5,777,350 disclosed LEDs comprising multiple layers of indium and gallium nitrides and p- and n-type AlGaN, which emit in the wavelength range of about 380 nm to about 420 nm. The active layer of such a LED may be doped with other materials to shift the LED peak emission within the UV-to-blue wavelength range. A LED having a peak emission in the blue light wavelengths was combined with a coating of a yellow light-emitting yittrium aluminum garnet phosphor activated with cerium (xe2x80x9cYAG:Cexe2x80x9d) to produce white light is disclosed in U.S. Pat. No. 5,998,925. Although a substantial portion of the need for white light devices may be filled by LED-based devices, the ability to combine a UV/blue LED with a phosphor has been limited because YAG:Ce has been the only known yellow light-emitting phosphor that is excitable by radiation in the blue range.
Therefore, there is a need to provide new phosphors that are excitable in the near UV-to-blue range and emit in the visible range. It is also desirable to provide novel phosphors that emit light in a broad wavelength range from blue green to red so that they may be combined with UV/blue LEDs to produce white light of high efficiency and/or high color rendering index (xe2x80x9cCRIxe2x80x9d).
The present invention provides phosphors that are excitable by radiation having wavelengths in the near UV-to-blue range (from about 315 nm to about 480 nm) to emit efficiently a visible light in a broad range of wavelengths from about 490 nm to about 770 nm having an emission peak in the green-to-yellow wavelength range. In general, the phosphors of the present invention are oxide solids containing at least terbium and at least one element selected from the group consisting of aluminum, gallium, and indium and are activated with at least one rare-earth metal ion selected from the group consisting of cerium, praseodymium, neodymium, samarium, europium, gadolinium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. Terbium may be substituted partially by at least one rare-earth metal selected from the group consisting of yttrium, lanthanum, gadolinium, samarium, and lutetium. The phosphors of the present invention have a garnet structure and a general formula of
(Tb1-x-yAxREy)3DzO12 
where A is a member selected from the group consisting of Y, La, Gd, and Sm;
RE is a member selected from the group consisting of Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, and combinations thereof; D is a member selected from the group consisting of Al, Ga, In, and combinations thereof; x is in the range from 0 to about 0.5, and y is in the range from about 0.0005 to about 0.2, and z is in the range from about 4 to 5. In one aspect of the present invention, 4 less than z less than 5.
According to another aspect of the invention, a method for producing a rare earth-activated terbium-containing garnet phosphor is provided and comprises the steps of: (1) providing stoichiometric amounts of oxygen-containing compounds of terbium, oxygen-containing compounds of at least one rare-earth metal selected from the group consisting of Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu; and oxygen-containing compounds of at least one member selected from the group consisting of Al, Ga, and In; (2) mixing together the oxygen-containing compounds to form a mixture; (3) optionally adding at least one fluxing compound selected from the group consisting of fluorides of Tb, Al, Ga, In, Y, La, Gd, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu in the mixture in a quantity sufficient to act as a flux; and (4) firing the mixture in a reducing atmosphere at a temperature and for a time sufficient to convert the mixture to a rare earth-activated terbium-containing garnet phosphor.
In another aspect of the invention, a first solution of stoichiometric amounts of oxygen-containing compounds of terbium; at least one other rare-earth metal selected from the group consisting of Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu; and at least one metal selected from the group consisting of Al, Ga, and In is precipitated in a basic solution to obtain a mixture of hydroxides of the metals. The mixture of precipitated hydroxides is calcined in an oxidizing atmosphere. The calcined material is further thoroughly mixed, and then fired in a reducing atmosphere at a temperature and for a time sufficient to convert the calcined mixture to rare earth-activated terbium-containing garnet phosphor. In one embodiment of this process, a fluoride of at least one metal selected from the group consisting of Tb, Al, Ga, In, Y, La, Ga, Sm, Ce, Pr, Nd, Eu, Dy, Ho, Er, Tm, Yb, and Lu into said first solution.
In still another aspect of the present invention, a light source emitting white light is provided and comprises a UV/blue LED, an amount of a rare earth-activated terbium-containing garnet phosphor having the formula (Tb1-x-yAxREy)3DzO12 wherein A, RE, D, x, y, and z are defined above. The phosphor is disposed adjacent to the UV/blue LED such that the phosphor absorbs at least a portion of the radiation emitted by the UV/blue LED to a visible light. The light emitted by the phosphor and a portion of the radiation emitted by the UV/blue LED are combined to produce a white light.